If you already have ANY kind of google account (gmail, google docs, adwords, adsense, google analytics) you just need to log into your account and look for the Google+ page link.
If you don’t have any existing google accounts, sign up for your Googe+ account here: https://plus.google.com/
The video below is a great introduction into how many of the Google+ features work.
1. Be sure to fully fill out your profile first
2. Invite your friends to join your circles